1. Field of the Invention
The present invention relates to a sheet conveying apparatus and an image forming apparatus.
2. Description of the Related Art
In a commercial printing business, Print on Demand (POD) by an image forming apparatus using electrophotography instead of using an offset printing machine has been provided for printing small lots of data, various types of data or variable data has been increasing. In order to meet this kind of need, registration on both surfaces is required for the image forming apparatus using electrophotography comparable to that of the offset printing machine.
There are two main reasons for causing a registration error occurring in both-sides printing, including registration error in the lateral and the vertical directions, and a skew error between a sheet and an image. Further, for an image forming apparatus including a heat fixing device, an image size error caused by expansion and contraction of the sheet is also a reason for registration error occurring in both-sides printing.
In order to automatically correct the registration error in both-sides printing caused by the image size error, it is required to use a technique to automatically and accurately measure the size of a sheet, the conveying distance of the sheet or the like. Thus, a technique to measure the length of the sheet by detecting passing of a front end and a rear end of the sheet and calculating the length of the sheet based on the period between the passing of the front end and the rear end of the sheet, or the like is known.
For example, according to Patent Documents 1 to 3, a sheet length measurement means is disclosed. The sheet length measurement means includes a rotation amount measurement means that measures a rotation amount of a length measuring roller which is rotated in accordance with a movement of a sheet or the like, and edge sensors provided before and after the length measuring roller to detect passing of the sheet. The sheet length measurement means measures the length of the sheet or the like in the conveying direction of the sheet based on the rotation amount of the length measuring roller and detections by the edge sensors.
However, when there is an eccentric amount of the length measuring roller, if the phases of the length measuring roller at a start timing and an end timing are different, an error may be caused in the measured sheet length.
Thus, according to Patent Document 4, a length measuring apparatus including a length measuring roller, a first upstream edge sensor, a second upstream edge sensor and a downstream edge sensor is disclosed. In the length measuring apparatus, a length of the sheet in the conveying direction is calculated by selecting a length among a first length of a sheet measured within a first detection period by the first upstream edge sensor and the downstream edge sensor, and a second length of a sheet measured within a second detection period by the second upstream edge sensor and the downstream edge sensor, which becomes closer to an integer multiple of the perimeter of the length measuring roller.
According to Patent Document 4, it is described that a measurement error in the measured sheet length obtained by using the length measuring roller caused by the eccentric amount of the length measuring roller can be reduced.
However, according to the length measuring apparatus disclosed in Patent Document 4, there may be a case when both the first length of the sheet measured within the first detection period by the first upstream edge sensor and the downstream edge sensor, and the second length of the sheet measured within the second detection period by the second upstream edge sensor and the downstream edge sensor do not become an integer multiple of the perimeter of the length measuring roller. In such a case, the measurement error in the measured sheet length obtained by using the length measuring roller caused by the eccentric amount of the length measuring roller cannot be reduced.